The Inconstant Moon

9/19/06 – 9/25/06
by C. Zaitz

I was in a class recently where we were given some questions on general science knowledge. One of the questions was, “explain why the moon has phases.” Inwardly I squealed with glee; I had trouble with two other quizzes I had taken that day, but I certainly can explain the moon’s phases.

It may seem like a general knowledge, but few people really know why the moon goes through phases. After twenty years of anecdotal experience in the planetarium, I have an opinion about why. Moon phases are taught in school at an age where it’s very difficult for kids to understand them. In third grade, most kids are still concrete learners and making the jump to an abstract view of the solar system is almost impossible. If they could hop on the Magic School Bus and go into space to watch the moon orbit the earth, they would have an accurate picture. But they cannot, and they have to rely on inadequate 2-D drawings and verbal explanations. Even 3-D models are not always helpful, since their inaccurate scales can introduce more misconceptions. In the absence of truly understanding these models, kids tend to make up their own explanations. Once these are made up, it’s very difficult in later life to counteract these ideas. So many adults still carry their childhood ideas about moon phases. Even some teachers!

I can tell you that moon phases are NOT caused by: 1. the earth’s shadow on the moon, 2. clouds covering part of the moon or 3. magic. The moon phases are caused by us seeing the illumination of the moon from different angles as it orbits us. Since the sun can only light half the moon at any given time, there are times when we look up at the moon and only see part of it illuminated. If it is opposite the sun from us, we can see the entire illuminated face. That’s Full Moon. But when the moon is in a different part of its orbit, we may see only a thin edge illuminated or an egg-shaped moon, not quite full. There are times when the moon lies somewhere between us and the sun. That is called New Moon; her illuminated side faces completely away from us. So where the moon is in its orbit around the earth determines what we see.

A logical question might be, “if the moon were between the earth and the sun, wouldn’t that make a solar eclipse?” Yes, it would, if the moon were precisely between the earth and sun. But the moon’s orbit is inclined to ours by about five degrees, making exact alignment rare. So rare, in fact, that the next solar eclipse visible from anywhere near Michigan will be on August 21, 2017. If you have family or friends in northern Oregon, Idaho, Wyoming, Nebraska, Missouri, Kentucky, Tennessee, North Carolina, Georgia, or South Carolina, now is a good time to secure their spare room. I anticipate a lot of excitement about a total solar eclipse in the heartland of the United States.

The dead, airless moon silently swings around the earth every month, completing its phases in 29.5 days. It waxes from the invisible New Moon to growing crescent, first quarter, bulging gibbous to Full Moon in about two weeks, and then it wanes through gibbous, last quarter, and finally crescent phase. The moon will be new on the 22nd, and every day after that you can begin to see the ever-so-lovely waxing crescent moon in the western sky at dusk.

Until next week, my friends, enjoy the view!